KVM: arm64: nv: Use reg_to_encoding() to get sysreg ID

Avoid open-coding and just use the helper to encode the ID from the
sysreg table entry.

No functional change intended.

Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230211190742.49843-1-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>